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Data Sufficiency Diagrams
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Unlike most question formats on standardized tests, the issue at stake on a Data Sufficiency question is what you do and don't know. It's not important to find the numerical answer to the question. Instead, you need to determine whether you could find such a single answer.
For this reason, the question in many Data Sufficiency items is vague. On some questions, this lack of clarity extends to a diagram, as well.
Diagrams appear with some frequency on Data Sufficiency questions, especially when the problem has to do with geometry. You'll also find diagrams--number lines, in particular--on questions dealing with other content areas, such as absolute value and inequalities.
The Facts About DS Diagrams
It's important to realize what these diagrams do and don't tell you. On a Data Sufficiency question, it's crucial that you assume the diagram is not to scale. Not only that, it might be radically misleading.
For instance, a triangle that appears to have a right angle may not be a right triangle, and vice versa. A number line may appear to have four equally spaced points that lie on it, but the points are probably not equally spaced.
Usually, all that DS diagrams tell you is how points are connected. Let's consider the examples I just mentioned. Why would the test show you a triangle if it's the completely wrong shape? They do so in order to refer to specific points or angles (like angle B or segment XY) with consistency. On a number line, if W, X, Y, and Z are on the line from left to right, you can trust that W < X < Y < Z, but you cannot take for granted that the points are spaced the way they appear to be spaced.
I like to think of Data Sufficiency diagrams as flexible. Imagine they are in a computer graphics editor, where you can click the mouse on one point and drag it, thus changing the shape of the figure.
Each time you get a piece of information (perhaps that AB = BC, or that angle B is a right angle), pretend you are editing the diagram in your head. Update the diagram, make all of the conclusions you can, but remember that the diagram can still be edited. On some questions, you'll have to change your image of the diagram three times: once for statement (1), once for statement (2), and once for the statements combined.
Flexibility is the key skill here. The GMAT isn't deliberately trying to confuse you with diagrams that aren't drawn to scale, but it isn't exactly helping, either. If you're prepared, you'll be able to overcome this obstacle with ease.
About the author: Jeff Sackmann has written many GMAT preparation books, including the popular Total GMAT Math, Total GMAT Verbal, and GMAT 111. He has also created explanations for problems in The Official Guide, as well as 1,800 practice GMAT math questions.
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The comprehensive guide to the GMAT Quant section. It's "far and away the best study material
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